It is known to measure the frequency of a signal by counting cycles during a predetermined time interval and calculating, from the count, a cycles-per-second figure. This method cannot, however, be used with signals whose frequency constantly changes, unless one wishes to measure the average or middle frequency.
It is also known to determine the period, or cycle of a signal by the counting of pulses. In order to obtain a sufficiently high resolution in the result, one needs clock pulses whose frequency is significantly higher than the frequency sought to be measured. In particular, high-resolution measurement of FM intermediate frequency signals yields values, for the known methods, that cannot be attained with known component parts. For instance, if such a signal in the intermediate frequency band (10.7 MHz) is to be detected with an accuracy of 12 bits, then for the least significant bit it must be possible to detect a period or cycle difference of 0.32 picoseconds (sec-.sup.-12). A measuring method that attains this precision is not realizable. With the required precision, even if the intermediate frequency signal is reduced into the base band, realizable values are not obtained. At least 614.4 MHz would be required as a counting frequency. This cannot be realized either, for the most various reasons.